It is especially difficult to maintain uniform consistency in the product intermittently dispensed from a soft-serve machine. As product is dispensed from one end of the freezing cylinder, new product ingredients are introduced into the other end of the freezing cylinder and must be congealed and homogenized. However, the product is only intermittenly dispensed from such machines and the duration of each dispensing operation and the intervals between dispensing operation varies widely. It is not only necessary to refrigerate the freezing cylinder to congeal the new product ingredients entering the freezing cylinder during the dispensing operation, but to also control refrigeration of the freezing cylinder between dispensing operations in a manner to compensate for heat losses to the ambient air and maintain the desired product consistency.
Various different control systems have heretofore been proposed for controlling refrigeration of the freezing cylinder and operation of the dasher in the cylinder for the purpose of controlling the consistency of the product being dispensed. Some control systems utilize temperature sensors to sense the temperature in the freezing cylinder and control operation of the refrigeration apparatus to maintain the temperature of the freezing cylinder in a range in which the product has the desired consistency. However, the temperature sensors only sense the temperature of the freezing cylinder at a localized area, generally adjacent the dispensing outlet and do not respond rapidly to changes in the product temperature remote from the outlet caused by new product ingredients entering the freezing cylinder.
Some other control systems sense the viscosity of the product in the freezing chamber, for example by sensing a torque required to drive the dasher in the freezing chamber. Of the control systems using viscosity type sensors, some drive the dasher continuously and turn the refrigeration to the freezing cylinder on and off under the control of the viscosity sensor. However, continued driving of the dasher after the product reaches its desired consistency not only wastes power but adversely affects the texture of some of the products due to the continued working of the semi-frozen product. The viscosity sensors do not accurately sense the product viscosity when the dasher is stopped and some control systems use the viscosity sensor to turn the refrigeration off when the product reaches a desired consistency, and then use a timer or some other program controller to turn the refrigeration and dasher drive back on at intervals between dispensing operations. However, the rate at which the product warms up in the freezing chamber when the refrigeration is shut off, varies with the ambient temperature and it is frequently desirable to operate the refrigeration mechanism either more or less frequently than that which would be indicated by a timer or similar mode control.